Caging Radioactive Gas

One of the challenges of nuclear energy is that the spent fuel emits dangerous, radioactive gasses. Not only is such a gas harmful to humans, but it makes storage and reprocessing difficult.

Now chemists at Sandia National Labs have found a way to isolate iodine gas from other molecules in nuclear fuel by trapping it in a kind of molecular cage. The metal-organic framework, known as an MOF, is a crystalline and porous substance in which a metal atom binds to organic molecules. The team got the idea for an MOF from zeolite, a common material used in industry as an absorbent (and shows up in laundry detergent as well). Zeolite is made of minerals that take on a porous structure, allowing it to absorb other molecules relatively easily. A form laced with silver atoms can take up radioactive iodine quite well, for example, because the silver and iodine combine to form silver iodide. But silver is expensive.

So the scientists turned to other materials. They found that if you put zinc in the center of a frame made of organic molecules (methyl imidazolate for aficionados) that the ores in it that are just about the size of an iodine molecule. That allows them to absorb the iodine and trap it in the molecular framework. What’s left can be incorporated into glass and stored safely. One big advantage to this kind of stuff is that it can be made into pellets or powders. The latter can blow around and that’s more efficient for cleaning up after a nasty spill. Pellets are good for getting iodine out of a controlled waste stream – the pellets retain a lot of surface area and are stable. This is the first time anyone has used a zeolite-like substance in this way, and it means that other, similar molecules could be made to absorb other dangerous chemicals as well.